I. Field of the Invention
The present invention relates to a thermal ink transfer laminate for transfer of an image from the laminate onto a receiving article.
II. Description of the Prior Art
Prior art thermal ink laminates consist typically of a support substrate of paper or plastic film coated with a heat sensitive ink coating which melts upon application of heat from a thermal printing element in contact with a side of the laminate opposite the coating. Upon melting, the ink becomes transferable onto a receiving article, thus forming transferred images on the receiving article. One form of related art is known as thermal paper. Thermal paper consists of a support substrate or plastic film coated with a heat sensitive ink coating. In thermal paper, local heat is applied directly to the coated surface, for example, by use of a thermal printing element in direct contact with the coating. The components in the ink coating react along the point of contact with the print element thereby forming images directly on the carrrier substrate itself. Although a heat sensitive ink coating is used in thermal paper, the image forms directly on the paper without being transferable onto another article. In contrast, applicant's invention is directed to thermal ink transfer laminates wherein images are transferred from the laminate to a receiving article. In applicant's product, unlike thermal paper, the images are formed on the transferable coating by localized heating from a thermal print element in contact with a side of the laminate opposite the coating, and the formed images are transferred onto a receiving article in contact with the coating.
Another related art is known as resistive-type ink transfer laminate formed of a heat sensitive ink coating on an electrically conductive support substrate, typically electrically conductive paper or plastic film. In the resistive-type laminate, the support substrate is electrically conductive so that as an electrical print element of two electrodes is applied to the side of the laminate opposite the ink coating, a current passes along the carrier surface between the electrodes. The passage of current results in localized heating which causes the ink coating to melt, thus forming images which are transferable onto a receiving article. In contrast, the thermal ink transfer laminate of the type of the present invention is not electrically conductive, but rather the support substrate need only be sufficiently thermally conductive to permit heat generated by a thermal print element to pass through the substrate and melt the heat sensitive ink coating on the opposite side.
U.S. Pat. No. 4,269,892 is illustrative of resistive types of ink transfer laminates wherein the substrate is electrically conductive. In this reference the substrate is a ribbon, typically of polyester, which contains dispersed therein electrically conductive particles of carbon black. The ribbon is overcoated with the thermal transfer ink layer which comprises a wax or a thermoplastic resin, carbon black pigment, and optionally a dye. As electrodes are applied to the side of the ribbon opposite the transfer ink layer, passage of current between the electrodes produces intense local heating along the contact points which causes transfer of ink to a receiving article. Since the ink transfer laminate disclosed in this reference is of the resistive type it does not bear on applicant's products which employ a nonelectrical conductive support substrate.
U.S. Pat. No. 3,871,065 is illustrative of thermal paper. In this reference, thermal paper is disclosed which is composed of a substrate, typically a paper sheet coated on one side with a heat sensitive ink coating. As a heated print element, for example a thermal printhead typically composed of heated dot matrices, is applied to the clean side of the substrate, the heat sensitive ink coating melts forming images directly on the coating. The invention described in this reference is directed to an improved heat sensitive ink coating. The ink sensitive coating described therein is of the reactive type, which is typically employed in thermal paper. In such reactive-type coatings there are two or more reactive color-forming components which react upon exposure to heat, thereby producing a color of high optical density. In addition to the reactive color-forming components, the ink formulation disclosed in this reference includes a binder, solvents, and an inhibitor which prevents reaction of the color-forming components prior to application of heat. One of the reactive color-forming components contains an iron salt of an aliphatic acid and the other contains a compound selected from the class of gallic acids and its derivatives. The invention is directed to an improvement in the heat sensitive ink coating layer, which improvement eliminates exfoliation by selection of a binder selected from the group of methyl cellulose and acetylcellulose having a melting point of not lower than about 180.degree. C. The ink coating formulation disclosed in this reference, which applies to thermal paper, does not bear on applicant's heat sensitive ink formulation since applicant does not employ reactive color-forming components or solvents.
U.S. Pat. No. 4,004,065 discloses a thermal paper having a heat sensitive ink layer of similar type to that disclosed in U.S. Pat. No. 3,871,065. The heat sensitive layer is an ink formulation composed of two reactive color-forming components, a binder, solvents, and an inhibitor. The color-forming components are composed of an iron salt of a higher fatty acid and gallic acid, which components react with each other to form a color upon exposure to heat. In addition to the color-forming agents, there is dispersed in the binder a stilbene series fluorescent dye, which inhibits the color-forming reaction prior to heat exposure. The binder is composed of at least one component selected from the group of hydroxypropyl cellulose and hydroxypropyl methyl cellulose. The heat sensitive ink formulation disclosed in this reference does not apply to applicant's formulation since applicant's formulation does not include reactive color-forming components. Furthermore, the images formed on the thermal paper are not transferable to a receiving article as in the case of applicant's invention.
U.S. Pat. No. 3,983,279 discloses a thermal paper of the above-type. However, the thermal paper described in this latter reference is composed of a support substrate, typically of paper which is coated on both sides with a heat sensitive color-forming layer. One of the heat sensitive layers (12) is nontransferable, and the other heat sensitive layer (13) is transferable onto a receiving article as illustrated in FIG. 3 upon application of heat to the substrate. Heat sensitive layer 13 is relevant to thermal ink transfer laminates, the subject of applicant's invention, only to the extent that it is a heat sensitive layer which transfers onto a receiving article upon application of heat to the support substrate. However, both heat sensitive layers disclosed in this reference, including the heat sensitive transfer layer 13 contain color-forming components which react with one another to produce color upon exposure to heat. In addition to the reactive color-forming components, the heat sensitive transfer layer 13 includes a binder of wax and resin, a plasticizer, and a solvent. The heat sensitive transfer layer does not bear on applicant's formulation, since applicant does not employ reactive color-forming components, and furthermore applicant's coating is a hot melt mix, that is, does not include a solvent.
A high speed thermal ink transfer recording substrate is disclosed in the publication, T. Ohno, M. Mizuguchi et al., "High Speed Thermal Ink Transfer Recording and Its Application", Journal of Applied Photographic Engineering, Vol. 7, pp. 171-174 (Dec. 1981). In this publication, a thermal ink transfer laminate is disclosed as composed of a thin support substrate of condenser paper or polyester film coated with a heat sensitive ink transfer coating. Upon contacting the substrate with a thermal printhead, typically a heated element having a dot matrix density of about 6.times.6 dots per square millimeter, the ink coating melts and forms an image which transfers onto a receiving article such as paper or plastic receptor. This publication discloses use of a condenser paper of 12 micrometer thickness having coated thereon a heat sensitive ink of basis weight of about 4 grams per sq. meter. The disclosed ink formulation is composed of a coloring pigment in a binder containing both a wax base and a resin. The publication indicates that the wax component enhances the adhesion and transferability of the ink coating, whereas the resin component enhances the durability of hardness of the transferred print image. The publication reports that high speed printing at speeds of less than 3 milliseconds per element pulse duration is best achieved if ink coatings having a melting point of about 65.degree. C. to 70.degree. C. are employed where the viscosity is low, on the order of 200 cps or lower at a temperature of about 90.degree. C. Physical properties of a preferred heat sensitive ink formulation are presented in Table I of the publication. The ink referenced therein has a melting point of about 76.degree. C., a melt viscosity of about 80 cps, and a coating weight of about 4 grams per sq. meter. The ink coating is transferred at an input printhead power of 0.7 watts, pulse duration as low as 1.5 milliseconds per element. There is no disclosure of the chemical nature of the resin or wax binder, or coloring agent, or any other additive in the ink formulation to produce a heat sensitive ink coating. Furthermore, there is no teaching of employing a coloring agent to control viscosity. One disadvantage with prior art thermal ink transfer substrates of the type disclosed in this publication is that the image does not transfer completely onto a receiving article, i.e. visible particles of ink are apt to be left behind on the support substrate in the regions of transfer. This can result in inconsistent or impaired image definition.
U.S. Pat. No. 3,616,015 is illustrative of heat transferable labels which are formed of a carrier support overcoated with a wax-based release layer and an ink layer thereon. Heat transferable labels of this type having formulations disclosed in this reference cannot be utilized with an electrically activated thermal print head of the dot matrix type. Instead it should be apparent from the reading of the reference as a whole that heat transferable labels are designed to transfer completely in large areas covered by the entire label itself rather than in tiny discrete dots which conform to the size and shape of corresponding heated dot elements of an activated thermal printhead. The heat transferable laminate disclosed in the '015 reference utilized heated platen rollers rather than a dot matrix type print head to effect transfer of the entire laminate from the carrier support. Additionally, heat transferable laminates of the type disclosed in this reference have release layers which melt at higher temperatures in order to accommodate transfer by rolling pressure from a heated surface at a temperature between about 250.degree. to 600.degree. F., for example 350.degree. F. (See column 3, lines 35 to 40.) The class of thermally activated ink transferable laminates of the type with which applicant's invention is directed thus must effect transfer by action of an electrically activated thermal print head, e.g. of the dot matrix type, which subject the transfer layer to much lower temperatures than the heated platen or roller. It should be evident that applicant's transfer laminate has transfer requirements with different objectives and conditions to satisfy than those disclosed in this reference. Consequently the formulations disclosed in this reference would not be workable in accomplishing applicant's objectives.
Accordingly, it is a principal object of the present invention to provide a thermal ink transfer laminate exhibiting improved image definition upon transfer of the formed images from the transfer laminate to a receiving article.
It is an important object that the formed images exhibit improved release characteristics permitting improved, complete transfer of the formed images from the transfer laminate to a receiving article.
It is another important object of the present invention to provide a thermal ink transfer laminate wherein the transferred images exhibit improved adhesion to the receiving article.